Acquired immunodeficiency syndrome (AIDS) is one of the most destructive epidemics in medical history. In 2009, the UNAIDS report estimated that 35 million people are living with human immunodeficiency virus (HIV) infection and AIDS, 25 million deaths have occurred, and 14 million children have been orphaned since the epidemic began in 1981. The discovery of HIV, the etiological agent for AIDS, led to the identification of a number of biochemical targets to combat this devastating disease. Among them, therapeutic inhibition of a proteolytic enzyme, HIV-1 protease, emerged as a critical drug-development target. Subsequent design and discovery of protease inhibitors (PIs) and their introduction into the highly active antiretroviral therapy (HAART), marked the beginning of a new era of management of HIV-1 infection and AIDS. HAART significantly improved the quality of life and life expectancy of patients. There is no cure for HIV/AIDS and long-term treatment has posed a serious challenge because of the emergence of multidrug-resistant HIV-1 variants. About 40-50% of those patients who initially achieved favorable viral suppression to undetectable levels experienced treatment failure. These drug-resistant HIV strains can be transmitted, raising further uncertainty with respect to future treatment options. In addition, PIs are faced with a number of serious limitations including, major toxicity, tolerance, and adherence to complex medical regimens. The development of a new generation of PIs effective against drug-resistant HIV and with minimum side effects, are vital to the future management of HIV/AIDS. Our collaborative research efforts to combat drug resistance, led to the development of darunavir which was first approved for treatment against drug-resistant HIV in June, 2006, and then received full approval for all HIV/AIDS patients including pediatric patients in December, 2008. While darunavir has become a front line therapy against HIV/AIDS, it is far from ideal as an effective long-term treatment option. During this project period, based upon X-ray crystal structures of complexes of darunavir or other PIs with HIV-1 protease, we designed and synthesized a diverse class of potent PIs with marked antiviral activity, and excellent drug-resistance profiles against multidrug-resistant HIV-1 strains. We have also developed tools and important 'backbone binding' design concepts to combat drug-resistance. Furthermore, we have discovered a number of small molecule nonpeptide structural leads for optimization. A recent inhibitor, GRL-0519, has consistently shown a 10-fold improvement of potency compared to darunavir against a panel of multidrug-resistant HIV-1 variants. This PI also exhibited 10-fold better dimerization inhibitory properties of HIV-1 protease. Our current proposed studies are now focused on design, synthesis, and evaluation of the next generation of PIs for clinical development. Our multidisciplinary research efforts integrate structure-based design, synthesis, protein-ligand X-ray crystallography, inhibition kinetics, molecular modeling, and in-depth virus and cell-biological studies.